The invention concerns a traveling wave tube wherein a cylindrical vacuum envelope surrounds a delay line. The vacuum envelope is tightly surrounded by a permanent magnet system formed of annular pole discs and respective magnetic rings arranged between the pole discs which are oppositely polarized in alternating fashion along an axial direction of the tube. The permanent magnet system is inserted into an outside envelope in form-fit fashion.
A traveling wave tube having such a permanent magnet system is disclosed in German Pat. No. 32 16 250, incorporated herein by reference.
This system is also referred to as a PPM focusing system (Periodic Permanent Magnetic focusing system). Previously known PPM systems used pole discs which are centered on the vacuum envelope of the traveling wave tube with a certain play. At the same time, both the ring magnets as well as the pole discs had to observe extreme parallelism. This system presents difficulties in view of the balancing function. In a PPM focusing system for traveling wave tubes, there is the problem of generating a sequence of alternating magnetic fields which can be easily balanced on the tube. Nonetheless, a stable behavior of the overall mechanical structure must be guaranteed, even given a greater thermal stress, that is both a continuous stress as well as an alternating stress. The previous plug-in technology also exhibits problems involving the fitting tolerances. In order to create a stable PPM focusing system, German Pat. No. 32 16 250 discloses that the pole discs can be soldered to corresponding spacing rings of nono-magnetic material to form a rigid, stable unit.
It is also known to glue together a PPM focusing system. The problem arises, however, that the tube focusing can fail due to an undefined gap formation between magnets and pole shoes produced by a sudden temperature change on the order of 150.degree. C. Such a large temperature modification in traveling wave tubes is required, for example, for use in cosmic space conditions, and is practically unavoidable, or can only be avoided at great technological expense. The traveling wave tube may become unusable due to the temperature shock. The cause is a spontaneous formation of radial cracks in the connection or gluing of the focusing system at the locations of lowest strength produced due to excessively great differences in the thermal expansion of different materials. This problem therefore particularly occurs because the magnet system is formed of a plurality of magnets and pole shoes, for example 45 magnets and 46 pole shoes, which are movably mounted on the delay line.